1. Field of the Invention
The present invention relates generally to the fields of molecular immunology and genetics. More specifically, the present invention relates to a novel .DELTA.Nur77 trangenic mouse.
2. Description of the Related Art
Clonal deletion and clonal anergy are the primary mechanisms for induction of self-tolerance in T cells (1). During thymic maturation, thymocytes bearing self-reactive T cell receptors (TCR) undergo clonal deletion and are eliminated by programmed cell death or apoptosis (2, 3). Thymocytes with intermediate to high density levels of expression of the TCR undergo negative selection at the CD4.sup.+ CD8.sup.+ (double positive) stage of thymocyte development (4-8). Down-modulation of the TCR, CD4 and/or CD8 molecules on the surface of T cells has been proposed as a mechanism for escape from negative selection (8-12). Thus, the TCR generates signals that are capable of mediating negative selection of thymocytes by clonal deletion. However, the signaling mechanisms required for negative selection of thymocytes remain unknown.
Several molecules and pathways known to be of importance in apoptosis have been described in the thymus, however, their contribution to clonal deletion and tolerance induction remains controversial (1). Although knockout of p53 leads to decreased sensitivity of murine thymocytes to radiation-induced apoptosis, negative selection remains intact (13-15). Fas is a cell surface receptor that mediates apoptosis by interaction with a specific ligand and is expressed on most murine thymocytes (16-19). Although mutant Fas antigen and Fas ligand cause autoimmune disease in lpr/lpr and gld/gld mice, respectively (18-20), no major negative selection defects have been found in lpr/lpr mice (21-25). Therefore, it is unlikely that Fas antigen is directly involved in negative selection in the thymus.
Bcl-2 prevents thymocyte apoptosis that is induced by radiation, steroids, and other chemicals (26-28). Expression of bcl-2 has been reported to be decreased in CD4.sup.+ 8.sup.+ thymocytes, but not in mature thymocytes, and has been proposed to play a role in inhibition of negative selection in the thymus (29). However, bc/-2 knockout mice do not exhibit excessive clonal deletion in the thymus (30) and, conversely, bcl-2 transgenic mice do not exhibit a major defect in negative selection or T cell tolerance (28, 31-33). Double transgenic bcl-2+D.sup.b /HY TCR mice show that constitutive expression of bcl-2 increases the survival of thymocytes in the absence of positive selection (34-36). The bcl-2 transgene also reduces the efficiency of negative selection, but the mature peripheral T cells which appear in increased numbers were not autoreactive. Thus, although bcl-2 can play a role in both positive and negative selection, tolerance is maintained by a mechanism that can bypass bcl-2.
NGFI-B/Nur77 is a growth factor-inducible member of the steroid/thyroid hormone receptor superfamily originally identified in nerve growth factor (NGF)-treated P12 pheochromocytoma cells (37) and in serum-stimulated fibroblasts (38). NGFI-B/Nur77 is transcriptionally regulated as an immediate-early gene and is rapidly activated by phosphorylation after stimulation with serum or nerve growth factor (39, 40). NGFI-B/Nur77 has a centrally located, highly conserved DNA binding domain containing two zinc-fingers and a transcriptional trans-activating domain (41-47). NGFI-B/Nur77 gene is expressed in thymic medulla and is rapidly upregulated in T cell hybridomas and thymocytes after treatment with anti-CD3 or anti-TCR and this expression has been correlated with induction of apoptosis (48, 49). Blocking of nerve growth factor I-B with either a dominant negative truncated (48) or antisense (49) NGFI-B/Nur77 gene prevented TCR/CD3 signaling-mediated apoptosis in T cell hybridomas.
There are at least two gene families with an identical nerve growth factor I-B response element (NBRE) AAAGGTCA (50). The first member of this family, referred to as Nur77 (mouse), nerve growth factor I-B (rat), and NAK-1 (human) peaks 1 hour after stimulation of the PEER T cell line. The second member, referred to as Nurr1 (mouse), RNR-1 (rat), and transcriptionally inducible nuclear receptor (human) peaks 24 hours after stimulation and correlates with apoptosis. Although Nur77 has been shown to be important in T cell signaling and apoptosis (48, 49), other signaling proteins can also contribute to T cell maturation and apoptosis. This was recently demonstrated in a Nur77 mutant mouse, in which T cells did not exhibit defective apoptosis after anti-CD3 crosslinking and exhibit normal T cell development and apoptosis in D.sup.b /HY TCR transgenic mice (51).
The prior art is deficient in the lack of a .DELTA.Nur77 trangenic mouse. The present invention fulfills this longstanding need and desire in the art.